1. Field of the Invention
The present invention relates to an image processing apparatus and method of controlling an image processing apparatus applicable to an imaging apparatus and the like which captures still images and moving images, and in particular, relates to an image processing apparatus and method of controlling an image processing apparatus capable of eliminating beat noise convoluted on captured visual signals.
2. Description of the Related Art
In recent years, advances have been made in the down-sizing and high-pixelization of solid state image sensors such as CCDs that are used in mobile phones and the like. This has resulted in an increase in the drive frequencies of imaging circuits that drive such solid state image sensors. In addition to a drive circuit for a solid state image sensor, an imaging apparatus includes a control system circuit for a CPU and the like, a memory circuit for a DRAM and the like, as well as a display circuit for an LCD and the like. Each of these circuits operates according to clock signals having mutually different high frequencies. With such an imaging apparatus in which a plurality of clock signals having different frequencies exists, there is an unignorable problem in that reduced board sizes due to down-sizing of the apparatus give rise to clock interference between the circuits. Examples of such interference include interference with a horizontal interval of a clock signal, interference between a horizontal interval of a clock signal and power wiring patterns, and interference between a horizontal interval of a clock signal and analog signals. An occurrence of such an interference of a clock signal causes frequency noise of the interfering clock signal to be convoluted on captured visual signals, resulting in a problem of deterioration of the quality of obtained image data.
As for conventional art, for example, Japanese Patent Laid-Open No. 7-67038 proposes an arrangement in which, for the purpose of extracting and canceling noise convoluted on a visual signal, a storage unit is provided which stores image data corresponding to one horizontal interval. The arrangement integrates optical black pixels in the vertical direction of a solid state image sensor over a horizontal interval and stores the integrated results in the storage unit, and subsequently eliminates convoluted noise by subtracting stored image data corresponding to one horizontal interval from effective pixel data.
As for other conventional art, for instance, Japanese Patent Laid-Open No. 2002-152600 proposes a technique for canceling beat noise signals. In this example, a multi-channel clamping circuit is provided which has a plurality of channels corresponding to the number of pixels equivalent to one cycle of a beat noise signal formed by two types of clock signals, and which sequentially integrates signals of each pixel in the optical black area in the capture for each of the plurality of channels. The method used in Japanese Patent Laid-Open No. 2002-152600 eliminates beat noise signals by subtracting this integrated value from input image data.
However, in the conventional example described in Japanese Patent Laid-Open No. 7-67038, since image data of a plurality of horizontal intervals are integrated as a signal corresponding to one horizontal interval, only noise occurring in the vertical direction is extracted. Thus, the conventional example is incapable of handling noise generated in an oblique direction.
In addition, Japanese Patent Laid-Open No. 2002-152600 does not take into consideration a method of processing in a case where the cycle of beat noise signals is not an integral multiple of pixel data. Therefore, the contents proposed in Japanese Patent Laid-Open No. 2002-152600 are insufficient in handling such a case. In practice, there are cases where the cycle of a beat noise signal or an initial phase value is a rational multiple of pixel data. Furthermore, the invention proposed in Japanese Patent Laid-Open No. 2002-152600 is incapable of eliminating a beat noise signal unless the cycle of the beat noise signal is identified in advance. Moreover, this arrangement of conventional art is configured to eliminate noise on a per-pixel basis with respect to image data from a solid state image sensor. However, elimination of beat noise signals cannot be performed on image data photographed by a multi-field read type solid state image sensor and framed on a memory.